System and method for testing shell and tube heat exchangers for defects

ABSTRACT

A magnetostrictive transducer assembly for generating a longitudinal elastic guided wave of a selected frequency and mode and for guiding the wave into an open end of a heat exchanger tube for testing the tube. The transducer assembly comprises a current-carrying coil of wire, a magnetostrictive material wrapped around the coil of wire, a mechanism for pressing the magnetostrictive material against an inner surface of the tube, and one or more biasing magnets placed in the vicinity of the current-carrying coil of wire and the magnetostrictive material.

BACKGROUND

Shell and tube type heat exchangers are commonly used in oil refineries,chemical process plants, and other large-scale applications because oftheir ability to handle high-pressure and high volume fluid flow. Suchheat exchangers typically consist of a large pressure vessel or shelland a number of tubes positioned inside the shell. One fluid runsthrough the tubes, and another fluid runs through the shell and over thetubes to transfer heat from or to the fluid in the tubes.

A non-linear tube heat exchanger is a particular type of shell and tubeheat exchanger that includes spiral-turned tubes rather than straightones. The spiral-turned tubes produce forced vortex fluid motion and/orother fluid agitation and swirl flow to enhance their thermal efficiencyand thus permit use of smaller heat exchangers when space is limited.

Heat exchangers with spiral-turned or straight tubes must be tested fordefects before they are placed in service and periodically thereafter toensure optimum performance and to prevent leakage from or into thetubes. This strict separation of fluids can be compromised by tubefailure, which may be caused by corrosion, metal erosion, or cracking.Tube failure is additionally problematic because it reduces the thermalefficiency of a heat exchanger and can impede fluid flow. Thereforeregular tube inspection and maintenance is desirable. Unfortunately, itis difficult and time-consuming to test shell and tube type heatexchangers, especially those with spiral-turned tubes.

SUMMARY

The present invention provides improved systems and methods for testingshell and tube type heat exchangers for defects. A method in accordancewith one embodiment of the invention broadly comprises the steps ofgenerating a longitudinal elastic guided wave of a selected frequencyand mode; guiding the wave into an open end of a heat exchanger tube;sensing a reflection of the guided wave from a defect in the tube;measuring a time duration between the generation of the guided wave andthe sensing of the reflection of the guided wave; and determining alocation of the defect in the tube based on the measured time duration.

A key aspect of the above-described method and other embodiments of theinvention is the use of selectively excited longitudinal guided wavesfor tube inspection rather than other types of elastic waves. Applicantexperimented with various different ultrasonic waves to test for defectsinside the tubes of non-linear heat exchangers but initially haddifficulty in getting the waves to traverse the non-linear geometries ofthe tubes over long distances. Applicant then discovered that such heatexchangers could be successfully tested with ultrasonic longitudinalguided waves generated and guided in a specific manner as described inmore detail below.

The present invention also provides unique transducer assemblies forimplementing the above described method and other embodiments of theinvention. One such transducer assembly is a magnetostrictive typetransducer that generates a longitudinal elastic guided wave of aselected frequency and mode and guides the wave into an open end of aheat exchanger tube for testing the tube. The transducer assembly maycomprise a current-carrying coil of wire; a magnetostrictive materialwrapped around the coil of wire; biasing magnets in the vicinity of thecurrent-carrying coil of wire and the magnetostrictive material; a probefor inserting the other components of the transducer assembly into anopen end of a tube to be tested. The current-carrying coil of wire maycomprise a dual-layer flexible coil made of copper-plated polymide andconfigured to operate at an excitation frequency of approximately 500kHz. The current-carrying coil of wire may also comprise two separatecoils that can be independently energized.

Applicant further discovered that the above-described transducerassembly operates more effectively when its magnetostrictive material isin firm contact with the spiral-turned tube in which it is positioned.Thus, embodiments of the probe may comprise a mechanism for pressing themagnetostrictive material against an inner surface of the spiral-turnedtube. In one embodiment, the mechanism comprises an expandable boot orair bladder. In other embodiments, the mechanism may comprise amechanically-actuated expander.

Applicant further discovered that the front tube sheet of shell and tubetype heat exchangers can interfere with the transmission and receipt ofguided waves. Thus, embodiments of the probe may have an elongatedsupport neck for inserting the coil of wire and the magnetostrictivematerial a distance beyond the tube sheet to avoid interference with thetube sheet.

This summary is provided to introduce a selection of concepts in asimplified form that are further described in the detailed descriptionbelow. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Other aspectsand advantages of the present invention will be apparent from thefollowing detailed description of the embodiments and the accompanyingdrawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Embodiments of the present invention are described in detail below withreference to the attached drawing figures, wherein:

FIG. 1 is a perspective view of an exemplary shell and tube type heatexchanger that may be tested with systems and methods of the presentinvention, with portions of the heat exchanger broken away to show aninternal tube bundle.

FIG. 2 is a fragmentary perspective view of a portion of a spiral-turnedtube bundle of the heat exchanger.

FIG. 3 is a block diagram of a testing system constructed in accordancewith one embodiment of the invention.

FIG. 4 is a block diagram of components of the control unit of thetesting system.

FIG. 5 is a block diagram of components of the transducer assembly ofthe testing system.

FIG. 6 is a perspective view of an embodiment of the wire coil of thetransducer assembly.

FIG. 7 is a front view looking into a heat exchanger tube showing oneembodiment of the transducer assembly inside the heat exchanger tube.

FIG. 8 is a vertical side sectional view of a heat exchanger tubeshowing one embodiment of the transducer assembly shown inside the heatexchanger tube.

FIG. 9 is a vertical side sectional view of a probe that may be used toinsert portions of the transducer assembly into an open end of a heatexchanger tube, with the probe shown in its retracted position.

FIG. 10 is a vertical side sectional view of the probe of FIG. 9 in itsexpanded position.

FIG. 11 is a phase velocity dispersion curve for a 0.75″ OD steel heatexchanger tube with 0.1″ wall thickness with the 500 kHz activation lineand fundamental longitudinal modes noted.

FIG. 12 is a group velocity dispersion curve for a 0.75″ OD steel heatexchanger tube with 0.1″ wall thickness with the 500 kHz activation lineand fundamental longitudinal modes noted.

FIG. 13 identifies the displacement component orientations for a heatexchanger tube.

FIG. 14 illustrates the displacement wave structures through thethickness of a tube wall for the points highlighted in FIGS. 11 and 12.

The drawing figures do not limit the present invention to the specificembodiments disclosed and described herein. The drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the invention.

DETAILED DESCRIPTION

The following detailed description of embodiments of the inventionreferences the accompanying drawings. The embodiments are intended todescribe aspects of the invention in sufficient detail to enable thoseskilled in the art to practice the invention. Other embodiments can beutilized and changes can be made without departing from the scope of theclaims. The following detailed description is, therefore, not to betaken in a limiting sense. The scope of the present invention is definedonly by the appended claims, along with the full scope of equivalents towhich such claims are entitled.

In this description, references to “one embodiment”, “an embodiment”, or“embodiments” mean that the feature or features being referred to areincluded in at least one embodiment of the technology. Separatereferences to “one embodiment”, “an embodiment”, or “embodiments” inthis description do not necessarily refer to the same embodiment and arealso not mutually exclusive unless so stated and/or except as will bereadily apparent to those skilled in the art from the description. Forexample, a feature, structure, act, etc. described in one embodiment mayalso be included in other embodiments, but is not necessarily included.Thus, the present technology can include a variety of combinationsand/or integrations of the embodiments described herein.

The present invention provides systems and methods for testing shell andtube type heat exchangers for cracks, internal protrusions, bends,corrosion, holes, and other defects. An exemplary heat exchanger thatmay be tested with the technology of the present invention isillustrated in FIG. 1 and represented broadly by the numeral 10. Theexemplary heat exchanger 10 is a shell-and-tube heat exchanger andincludes an elongated shell 12 having a front end 14, an opposed end 16,and an open interior volume 18. The shell 12 is of a generallycylindrical configuration, although it may have other shapes. The shell12 is formed of a metal, polymer, or other material that is generallyinert to the fluid within the shell 12 and is able to withstand thepressures and temperatures within the shell 12 during operation of theheat exchanger 10.

An inlet nozzle 20 extends from the shell 12 at the front end 14 forintroducing a shell-side fluid into an interior volume 18 of the shell12. An outlet nozzle 22 extends from the shell 12 for removing theshell-side fluid from the interior volume 18 of the shell 12. The outletnozzle 22 may be positioned at the opposite end 16 of the shell 12 fromthe front end 14 at which the inlet nozzle 20 is positioned. The outletnozzle 22 may also be positioned at the front end 14 with the inletnozzle 20 and a longitudinally-extending baffle may be positioned withinthe interior volume 18 of the shell 12. The longitudinally-extendingbaffle forces the shell-side fluid to flow from the inlet nozzle 20 tothe opposite end 16 of the shell 12 before reversing direction to flowon the opposite side of the baffle back to the front end 14 where itexits the interior volume 18 of the shell 12 through the outlet nozzle22. The inlet nozzle 20 and the outlet nozzle 22 typically extendradially from the shell 12, but they may extend from the shell 12 inother orientations, such as tangentially.

The illustrated heat exchanger 10 is a single pass type exchanger withan inlet channel or head 24 defining an interior plenum. An inlet nozzle26 for the tube-side fluid is positioned to close the open front end 14of the shell 12. An outlet channel or head 28 defining an interiorplenum and having an outlet nozzle 30 for the tube-side fluid ispositioned to close the open end 16 of the shell 12.

The heat exchanger 10 may instead be a two pass tube-side type exchangerwhere the inlet head 24 and outlet head 28 are both positioned at thefront end 14 of the shell 12 and the other end 16 of the shell isclosed. The inlet nozzle 26 and outlet nozzle 30 extend along thelongitudinal center axis of the shell 12 in the illustrated embodiment,but they may extend in other orientations, such as perpendicularly tothe longitudinal center axis of the shell 12.

A tube bundle 32 is positioned in the open interior volume 18 of theshell 12 and comprises a plurality of hollow, elongated tubes 34 thatextend in a parallel and spaced-apart relationship to each other and arepositioned in a preselected pattern. Each of the tubes 34 has an openfirst end 36 for entry of a tube-side fluid for flow within the tube 34along a longitudinal length of the tube 34 and an opposite open secondend 38 for the first fluid to exit the tube 34. The tubes 34 may beformed from thermally-conductive, corrosion-resistant materials, such asvarious metals, including copper alloy, stainless steel, carbon steel,non-ferrous copper alloy, Inconel alloys, nickel, Hastelloy alloys, andtitanium.

The present invention is particularly useful for testing non-linear heatexchanger tubes such as the tubes 34A illustrated in FIG. 2. Such tubes34A may be, for example, spiral-turned double radius oval tubes that arewelded or otherwise attached by their ends to tube sheets describedbelow. Such heat exchanger tubes produce forced vortex fluid motionand/or other fluid agitation and swirl flow to enhance their thermalefficiency to permit use of a smaller heat exchanger when space islimited.

Returning to FIG. 1, the tube bundle 32 may include a plurality ofplate-like baffles 40 positioned at spaced apart positions along thelongitudinal length of the tubes 34. The baffles 40 function to redirectthe flow of the shell-side fluid as it flows exteriorly of the tubes 34.The baffles 40 also serve to support and maintain the desiredpositioning of the tubes 34. As best shown in FIG. 4, each of thebaffles 40 has individual openings through which the tubes 34 extend.The openings are sized slightly larger than the tubes 34 to permit thetubes 34 to be longitudinally inserted through the openings whileminimizing the amount of the shell-side fluid that can pass through theopenings.

The tube bundle 32 also includes at least one tube sheet 42 that ispositioned at the front end 14 of the shell 12 and separates the openinterior volume 18 of the shell 12 from the interior plenum of the inlethead 24. The tube sheet 42 is normally disc-shaped with a perimeter thatseals against the inner surface of the shell 12 in a conventionalfashion. The tube sheet 42 includes a plurality of holes that extendbetween its opposing faces. The first ends 36 of the tubes 34 areinserted into and secured within the holes of the tube sheet 42. If thetubes 34 are U-shaped, the second ends 38 of the tubes 34 are insertedinto and secured within other holes of the tube sheet 42. In theillustrated embodiment in which the tubes 34 are straight, a second tubesheet 44 is positioned at the opposite end 16 of the shell 12 andseparates the open interior volume 18 of the shell 12 from the interiorplenum of the outlet head 28. The second ends 38 of the tubes 34 areinserted into and secured within the holes that extend through thesecond tube sheet 44.

In use, shell-side fluid is introduced through the inlet nozzle 20 intothe interior volume 18 within the shell 12 of the heat exchanger 10. Theshell-side fluid travels through a sinusoidal path as it travels alongthe length of the shell and navigates through the cutouts in the baffles40. The shell-side fluid is then removed from the interior volume 18 ofthe shell 12 through the outlet nozzle 22.

The tube-side fluid is introduced through the inlet nozzle 26 into theinterior plenum of the inlet head 24. The tube-side fluid is thendistributed to the first ends 36 of the tubes 34 and flows along thelength of the tubes 34 before exiting the second ends 38 of the tubes34. The tube-side fluid then enters the interior plenum of the outlethead 28 before exiting the heat exchanger 10 through the outlet nozzle30. As the shell-side and tube-side fluids travel within the heatexchanger 10, heat transfer occurs from one fluid to the other.

The above-described heat exchanger tubes 34, 34A and similar tubes maybe inadvertently formed with cracks, internal protrusions, bends, andother defects that inhibit fluid flow in the tubes. Likewise, the tubesmay develop cracks, holes, bends, and other defects during use. Thepresent invention provides methods and systems for testing for suchdefects so that detected defects can be fixed or otherwise remedied.

A method in accordance with one embodiment of the invention broadlycomprises the steps of generating a longitudinal elastic guided wave ofa selected frequency and mode; guiding the wave into an open end of aheat exchanger tube; sensing a reflection of the guided wave from adefect in the tube; measuring a time duration between the generation ofthe guided wave and the sensing of the reflection of the guided wave;and determining a location of the defect in the tube base on themeasured time duration.

An important aspect of the present invention is the use of ultrasoniclongitudinal guided waves of a selected mode and frequency. Applicantevaluated the dispersion of ultrasonic longitudinal waves in straighttubing and then evaluated the geometries of non-linear tubing todetermine the ideal mode and frequency range for generating and guidingultrasonic longitudinal waves in non-linear tubing. In some embodiments,the guided waves are in a frequency range between 20 kHz and 20 MHz.

Due to the dominant in-plane displacement of the longitudinal waveswithin certain frequency ranges, applicant discovered they are capableof propagating for long distance in spiral-turned tubes withoutsignificant mode conversion, wave deflection, or energy reflection.Applicant also determined that the longitudinal wave modes in theappropriate frequency ranges are sensitive to small defects of as littleas 1.6% tube cross-sectional area (CSA), and are capable of inspectingbeyond U-bends, and show minimal interaction with tube bands and at thecontact points between tubes.

Applicant developed phase and group velocity dispersion curves for anumber of sample heat exchanger tubes. The ends of a spiral-turned tubeare not twisted, hence allowing selection of ultrasonic energy from thatpoint into the spiral-turned tube using calculations developed forstraight tubing, as shown for one example of a 0.75″ OD steel tube witha 0.1″ wall thickness in FIG. 11. Calculation procedures to obtain thephase and group velocity dispersion curves and appropriate wavestructures in pipes and tubes can be found in many different sources.

Exemplary phase and group velocity dispersion curves for a 0.75″ ODsteel tube with a 0.1″ wall thickness are shown in FIG. 11 and FIG. 12,in which the L(0,1) and L(0,2) modes at 500 kHz frequency are noted. Theother modes present are higher-order flexural (i.e. non-axisymmetric)guided wave modes associated with either the fundamental L(0,1) orL(0,2) axisymmetric wave modes. The displacement component orientationsand the wave structures corresponding to the L(0,1) and L(0,2) wavesmodes at 500 kHz in this tube are shown in FIG. 13 and FIG. 14,respectively.

At 500 kHz, the L(0,2) mode has dominant in plane displacement in the Uzdirection compared to the out of plane component in the Ur direction. Atthis frequency, the phase velocity is around 5500 m/sec, and the groupvelocity is around 5100 m/sec. From the selected point and thedetermined velocities, transducer assembly designs were established toefficiently excite and receive such wave modes at this frequency.

The inspection methodology employed in embodiments of the presentinvention is pulse-echo, in which the excited wave, upon encountering adefect in a tube, is partially reflected back to the transducerassembly, which then acts as a receiver to detect the defect. Using theknown group velocity of the guided waves in the tube, the axial locationof the defect can be determined.

The present invention also provides unique testing systems forimplementing the above described methodology and other embodiments ofthe invention. A testing system 100 constructed in accordance with anembodiment of the invention is shown in FIG. 3 and broadly comprises atransducer assembly 102 and a control unit 104. The transducer assembly102 generates longitudinal elastic guided waves of a selected frequencyand mode and guides the waves into an open end of one of heat exchangertubes 34, 34A for testing the tube. The control unit 104 includes one ormore electronic components that facilitate signal generation, dataacquisition, and signal processing.

The transducer assembly 102 is preferably a magnetostrictive typetransducer with thin, flexible components. This allows the transducerassembly 102 to be fitted onto an expandable probe head and insertedinto a heat exchanger tube as described below. Such transducerassemblies have the additional advantages of being relatively powerfuland capable of exciting both longitudinal and torsional guided waves ifproperly designed.

An embodiment of the transducer assembly 102 is illustrated in FIG. 5and comprises a current-carrying coil of wire 106; a magnetostrictivematerial 108 wrapped around the coil 106; one or more biasing magnets110, 112 placed on opposite ends of the coil 106 and themagnetostrictive material 108; and a probe 114 for inserting the coil ofwire 106, magnetostrictive material 108, and biasing magnets 110 into aheat exchanger tube 34, 34A.

In more detail, the current-carrying coil of wire 106 may comprise adual-layer flexible coil made of copper-plated polymide and configuredto operate at an excitation frequency of approximately 500 kHz. Otherembodiments of the coil may operate at excitation frequencies between 20kHz and 20 MHz. The current-carrying coil of wire may also comprise twoseparate coils that can be independently energized. En exemplary coil ofwire is shown in FIG. 6.

The wire coil 106 may include several comb type elements, the spacing ofwhich is equal to the slope from the origin to the phase velocity andfrequency point of the desired mode on the phase velocity dispersioncurves. This spacing is equal to the wavelength of the guided wave. Thegroup velocity allows determination of the axial distance to a defect ina tube in a pulse-echo configuration based on the travel time of thewave energy reflected from the defect.

In some embodiments, two or more wire coils 106 may be employed toachieve cancellation of the reverse-traveling wave. This is accomplishedby either physically spacing the coils with a particular separation orby applying particular time delays to the signal to one of the coils ina manner that the reverse-propagating waves from both coils interact outof phase and thus superimpose to suppress one another while theforward-propagating waves superimpose to reinforce one another. Separatepulser and receiver coils may be utilized on a single probe to improvethe signal-to-noise ratio and/or to reduce the uninspectable “dead zone”immediately in front of the transducer. Multiple transducer coils on asingle probe also facilitate the collection of guided wave data over agreater frequency range than could be achieved with a single-coildesign.

The magnetostrictive material 108 is placed over the coil 106 andconverts magnetic energy created by the interaction of the coil 106 andthe permanent magnets 110 to kinetic energy that excites guided waves ina heat exchanger tube 34, 34A. The magnetostrictive material 108 may beFeCo (iron cobalt) or Ni (nickel). The guided waves are generated by aninteraction of the static biasing magnetic field from the biasingmagnets 110 and the alternating magnetic field induced by thecurrent-carrying traces on the wire coil 106.

The biasing magnets 110 are preferably permanent magnets but may also beelectromagnets. The size and design of the magnets 110 and the coil 106determines the guided wave modes and frequencies that can be generatedby the transducer assembly and detected by the control unit 104.

During wave generation, the magnetostrictive material 108 undergoes atime-varying strain in accordance with the frequency of the signal sentto the wire coil 106. This strain, if coupled properly to the innerdiameter of a tube 34, 34A, will excite guided waves in the tube, whichwill propagate and subsequently generate reflected wave energy uponinteraction with defects in the tube. These reflected waves propagateback toward the transducer assembly 102 and, if the transducer assemblyis coupled properly to the tube wall, will induce a strain in themagnetostrictive material 108 that will subsequently induce a current inthe wire coil 106 that is detected by the control unit 104 or otherinspection system.

The invention also requires proper alignment of the biasing magnets 110,wire coil 106, and magnetostrictive material 108, particularly at theedges of the latter two. If proper alignment is not achieved, thesignal-to-noise ratio is reduced and the uninspectable “dead zone” infront of the transducer is enlarged. These effects are due to unintendedexcitation of non-axisymmetric flexural waves and/or circumferentialwaves. Thus, the probe 114 ensures that the other components of thetransducer assembly are properly aligned and positioned within a tube34, 34A as shown in FIGS. 7 and 8.

Due to the dominant in-plane axial displacement component of thelongitudinal wave modes and the orientation and location of the probe114 inside the tube, shear coupling is required between themagnetostrictive material 108 and the inner diameter of the tube 34,34A. This coupling may be achieved with sufficient mechanical pressure,viscous shear couplant, a dry couplant such as neoprene, or acombination of these methods. Mechanical coupling alone may besufficient for guided wave excitation and detection, but the addition ofa shear couplant 116 shown in FIG. 7 substantially improves thesignal-to-noise ratio and the repeatability and reliable of theinspection results.

The above-described shear coupling may also be achieved mechanicallywith a mechanism for pressing the magnetostrictive material 108 againstan inner surface of the spiral-turned tube 34A. One such mechanismcomprises an expandable boot or air bladder 116 as shown in FIG. 8. Theboot or air bladder may be formed in nitrile tubing or other materials.

In other embodiments, the probe 114 may be configured to expand tofacilitate mechanical coupling of the magnetostrictive material 108 tothe tube wall so as to achieve the above-described shear coupling. Thisprobe configuration also allows for ease of insertion and removal of theprobe from tubes of various inner diameters.

An embodiment of an expandable probe 114 is shown in FIGS. 7, 9, and 10and includes a neck 117 on which the wire coil 106, magnetostrictivematerial 108, and biasing magnets 110 may be placed. The neck includesthree sections 118, 120, 122 that can be shifted by a shiftable taperedrod 124 between a retracted or unexpanded position shown in FIG. 9 andan expanded position shown in FIG. 10. The inner surfaces of the clampsections 118, 120, 122 have tapered walls 126 that mate with a taperedend of the rod 124. When the rod 124 is moved forward (from left toright from the perspective of FIGS. 9 and 10), it pushes the sections118, 120, 122 outwardly to press the magnetostrictive material 108 andwire coil 106 against the inside of the tube 34, 34A. Conversely, whenthe rod is moved rearward (right to left in FIGS. 9 and 10), it allowsthe sections 118, 120, 122 to retract so that the probe may be removedfrom the tube. The movement of the tapered rod may be provided byvarious means, including but not limited to, hydraulic, pneumatic,electric, or mechanical actuators. In order to easily retract theexpanded probe, a return mechanism may be included in the probe, such asa return spring.

Applicant further discovered that the front tube sheet 42 of the heatexchanger 10 can interfere with the transmission and receipt of guidedwaves. Thus, the support neck 117 may be several inches long forinserting the coil of wire 106 and the magnetostrictive material 108 adistance beyond the tube sheet 42 to avoid such interference. In oneembodiment, the support neck is approximately 12 inches long.

In some embodiments of the invention, the probe 114 may be designed withan interchangeable head to which the other components of the transducerassembly may be affixed. The benefit of such interchangeable heads isthat they would more easily allow for the inspection of a wide range oftube sizes.

In another embodiment, an electrothermal heater may be incorporated intothe probe 114 to allow for heating of the couplant 116 to control itsviscosity and facilitate probe extraction in cold conditions. The heatermay be a resistive heating coil printed on a flexible PCB circuit or anyother type of heater.

The control system 104 controls operation of the transducer assembly 102and detects and analyzes waves reflected from defects in the tube. Asshown in FIG. 4, one embodiment of the control system includes a signalgenerator/receiver 130, a processor 132, memory 134, and a userinterface 136.

The signal generator/receiver 130 generates the signals delivered to thewire coil 106 used to create the guided elastic waves. The signalgenerator/receiver also senses signals created by the wire coil 106 as aresult of reflections of the guided waves. The signal generator/receiver130 may include any conventional electronics capable of generating therequired signals.

The processor 132 controls the signal generator/receiver 130 andmeasures the time duration between the generation of a guided wave andthe receipt of a reflection to determine the location of defects in thetube. The processor may be part of a custom control device or may be acomponent of a conventional computer.

The user interface 136 allows an operator to initiate a testingprocedure and control the processor 132 and may include any conventionalbuttons, switches, touchscreen displays, etc. The control system 104 mayalso include, or be coupled with, a display for displaying results of aheat exchanger testing procedure.

The control system 104 may employ frequency sweeping to gatherultrasonic guided wave data over a range of frequencies so that afast-frequency analysis color map may be generated to aid an inspectorin signal interpretation and improve the probability of detection forcertain defects.

Aspects of the invention may be implemented with one or more computerprograms stored in or on the memory 134 or other memory residing on oraccessible by the processor 132. Each computer program preferablycomprises an ordered listing of executable instructions for implementinglogical functions in the processor. Each computer program can beembodied in any non-transitory computer-readable medium for use by or inconnection with an instruction execution system, apparatus, or device,such as a computer-based system, processor-containing system, or othersystem that can fetch the instructions from the instruction executionsystem, apparatus, or device, and execute the instructions. In thecontext of this application, a “computer-readable medium” can be anynon-transitory means that can store the program for use by or inconnection with the instruction execution system, apparatus, or device.The computer-readable medium can be, for example, but not limited to, anelectronic, magnetic, optical, electro-magnetic, infrared, orsemi-conductor system, apparatus, or device. More specific, although notinclusive, examples of the computer-readable medium would include thefollowing: an electrical connection having one or more wires, a portablecomputer diskette, a random access memory (RAM), a read-only memory(ROM), an erasable, programmable, read-only memory (EPROM or Flashmemory), an optical fiber, and a portable compact disk read-only memory(CDROM).

Although the invention has been described with reference to thepreferred embodiment illustrated in the attached drawing figures, it isnoted that equivalents may be employed and substitutions made hereinwithout departing from the scope of the invention as recited in theclaims.

Having thus described the preferred embodiment of the invention, what isclaimed as new and desired to be protected by Letters Patent includesthe following:
 1. A method of testing a heat exchanger tube for defects,the method comprising: generating a longitudinal elastic guided wave ofa selected frequency and mode with a magnetostrictive transducerassembly; guiding the elastic guided wave into an open end of the tube;sensing a reflection of the elastic guided wave from a defect in thetube; measuring a time duration between generation of the elastic guidedwave and the sensing of the reflection of the elastic guided wave; anddetermining a location of the defect in the tube based on the measuredtime duration.
 2. The method as set forth in claim 1, wherein themagnetostrictive transducer assembly is placed in an open end of thetube.
 3. The method as set forth in claim 1, wherein a frequency andmode of the guided wave is selected according to a characteristic of thetube, and wherein the selected frequency and mode of the longitudinalguided wave are generated by specially-designed transducers.
 4. Themethod as set forth in claim 1, wherein the longitudinal elastic guidedwave is generated with a magnetostrictive transducer assembly placed inthe open end of the tube, the magnetostrictive transducer assemblycomprising a current-carrying coil of wire, a magnetostrictive materialwrapped around the coil of wire, a mechanism for pressing themagnetostrictive material against an inner surface of the tube, and oneor more biasing magnets placed in the vicinity of the current-carryingcoil of wire and the magnetostrictive material.
 5. The method as setforth in claim 4, wherein the mechanism for pressing themagnetostrictive material against an inner surface of the tube comprisesan expandable boot or air bladder.
 6. The method as set forth in claim4, wherein the mechanism for pressing the magnetostrictive materialagainst an inner surface of the tube comprises a probe with amechanically-actuated expander.
 7. The method as set forth in claim 4,wherein the current-carrying coil of wire comprises a dual-layerflexible coil made of copper-plated polymide and configured to operateat an excitation frequency of approximately 500 kHz.
 8. The method asset forth in claim 4, wherein the current-carrying coil of wirecomprises two separate coils that can be independently energized.
 9. Themethod as set forth in claim 4, wherein the magnetostrictive materialcomprises a strip of iron cobalt material.
 10. The method as set forthin claim 4, wherein guided wave data is collected and analyzed over arange of frequencies.
 11. A transducer assembly for generating alongitudinal elastic guided wave of a selected frequency and mode andfor guiding the wave into an open end of a heat exchanger tube fortesting the tube for defects, the transducer assembly comprising: acurrent-carrying coil of wire, a magnetostrictive material wrappedaround the coil of wire, a mechanism for pressing the magnetostrictivematerial against an inner surface of the tube, and one or more biasingmagnets placed in the vicinity of the current-carrying coil of wire andthe magnetostrictive material.
 12. The transducer assembly as set forthin claim 11, wherein the mechanism for pressing the magnetostrictivematerial against an inner surface of the tube comprises an expandableboot or air bladder formed of nitrile tubing.
 13. The transducerassembly as set forth in claim 11, wherein the mechanism for pressingthe magnetostrictive material against an inner surface of the tubecomprises a mechanically-actuated expander.
 14. The transducer assemblyas set forth in claim 11, wherein the current-carrying coil of wirecomprises a dual-layer flexible coil made of copper-plated polymide andconfigured to operate at an excitation frequency of approximately 500kHz.
 15. The transducer assembly as set forth in claim 11, wherein thecurrent-carrying coil of wire comprises two separate coils that can beindependently energized.
 16. The transducer assembly as set forth inclaim 11, wherein the magnetostrictive material comprises a strip ofiron cobalt material.
 17. The transducer assembly as set forth in claim11, wherein the transducer assembly further comprises an elongatedsupport neck for inserting the current-carrying coil of wire and themagnetostrictive material a distance within the open end of the tube.18. The transducer assembly as set forth in claim 11, wherein thetransducer assembly includes a heating unit.
 19. The transducer assemblyas set forth in claim 11, wherein the transducer assembly comprises oneor more interchangeable transducer assembly heads having variousdimensions and transducer design characteristics.
 20. A system fortesting a spiral-turned tube of a heat exchanger for defects, the systemcomprising: a magnetostrictive transducer assembly for generating alongitudinal elastic guided wave of a selected frequency and mode andfor guiding the wave into an open end of the spiral-turned tube, thetransducer assembly comprising— a current-carrying coil of wire, amagnetostrictive material wrapped around the coil of wire, a mechanismfor pressing the magnetostrictive material against an inner surface ofthe spiral-turned tube, and one or more biasing magnets placed in thevicinity of the current-carrying coil of wire and the magnetostrictivematerial; and a control unit for sensing a reflection of the elasticguided wave from a defect in the spiral-turned tube and for measuring atime duration between generation of the guided wave and sensing of thereflection of the guided wave for determining a location of the defectin the spiral-turned tube base on the measured time duration.